Multiple COVID-19 Strains Carried By Travelers To Blame For Uptick In Northern California Infections

CALIFORNIA — The state of California has been hit especially hard by the coronavirus. But, why the Golden State? According to a new study, the epidemic being seen today in California was caused by multiple COVID-19 strains entering the state at around the same time. Most of these arrivals were facilitated by both state-to-state and international travel.

The study’s authors say their work makes a strong case for continued social distancing and travel restrictions in the United States.

To date, the state of California had recorded more than 131,000 confirmed coronavirus cases. Despite this exorbitant number of infections, there’s been a lack of real-time genomic epidemiology data. So, the study’s authors decided to use a new method called MSSPE that assembles viral genomes directly from clinical samples. Essentially, this approach allows researchers to take coronavirus samples from California patients and investigate each strain’s similarities and differences.

All of the examined samples were taken from northern California patients who had been infected through mid-March of this year.

In total, 36 samples collected from patients spanning nine different California counties were included in the research. All of those patients had been aboard the Grand Princess cruise ship during a recent voyage that resulted in a COVID-19 outbreak.


All of the viral samples’ genomes were sequenced and then placed on a phylogenetic tree. This process revealed that the viral genomes collected from California patients were diverse and spread all over the evolutionary tree of SARS-CoV-2. Some strains originated in Europe and New York, while others seem to date back to China. Additionally, many of the infected cruise passengers seem to have come into contact with a coronavirus strain from Washington state.

However, the results as a whole indicate that no one viral strain is dominating infections in northern California. This, according to the study’s authors, indicates that rates of community transmission are actually quite low. They believe most new infections are being spread by out-of-state and out-of-county travelers.

The research team concludes that their study confirms “robust insights into COVID-19 transmission are achievable if virus genomic diversity is combined and jointly interpreted with detailed epidemiological case data.”

The study is published in Science.

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